1. Field of the Invention
The present invention is directed to photoinitiated cationic epoxy compositions having good impact strength and toughness, as well as low color after curing.
2. Brief Description of Related Technology
Photoinitiated, cationically curable adhesive compositions based on epoxy resins have many benefits and advantages desirable to end user consumers. For example, such compositions provide cured products having physical properties comparable to those provided by thermosetting compositions, are less inhibited by oxygen than photoinitiated free radical curable compositions (resulting in faster and more thorough cure), and exhibit less shrinkage during cure than otherwise comparable photoinitiated free radical curable compositions (such as those based on acrylic chemistry).
Photoinitiated cationic adhesive compositions have, therefore, been used in various well-known applications such as liquid crystal displays and as laminates in digital video discs. However, current photoinitiated cationic adhesive compositions exhibit color formation after curing, such as a yellowing or browning of the composition. Moreover, such photoinitiated cationic adhesive compositions are typically further subjected to thermal processing, such as during a subsequent heat curing step (i.e. a “post-cure”) to ensure full curing of the composition, or other heat treatment of the composition for certain applications, depending on the intended use. Such thermal processing typically enhances the color formation, resulting in further yellowing or browning of the composition. In many applications, this color formation is undesirable, thereby reducing the usefulness of such adhesive compositions for certain applications, for example in the medical industry, such as adhesives for medical devices.
U.S. Pat. No. 6,777,460 to Palazzotto et al. discloses curing agents for cationically curable compositions which reduce coloring of the compositions. The curing agents include combinations of various photochemically active and non-photochemically active salts.
Thus, there has been an ongoing desire to find photoinitiated adhesive compositions that not only have good impact strength and toughness, but that also exhibit little to no color change after cure and/or after thermal exposure.